1. Field of the Invention
The invention relates to an optical imaging system with a variable focal length, more particularly a zoom lens for photographic cameras.
2. Description of the Prior Art
A hitherto known imaging system with a variable focal length has in its simplest form two lenses, the lens interval being varied along the optical axis in order to set the focal length and the focusing of one lens being effected in a definite relationship to the focusing of the other lens. This relationship is expressed by the following equations: ##EQU1## where L.sub.1 is the distance of the front lens from the fixed focal plane, s* is the distance of the rear lens from the fixed focal plane, f.sub.1 * is the image-side focal length of the first lens, f.sub.2 * is the image-side focal length of the second lens, and f.sub.12 * is the resultant focal length of the optical system.
In FIG. 1 of the drawing, this relationship is shown graphically, in which the distance of each lens from the focal plane L.sub.1 and s* are plotted as functions of the resultant focal length f.sub.12 *. It is readily apparent that the rear lens is to be focused linearly with the desired resultant focal length f.sub.12 *, and that the front lens is located in accordance with a second order curve. If this relationship is properly maintained, the position of the focal plane remains constant with any resultant focal length.
In the prior optical imaging system of this kind, cams corresponding to equations (1) and (2) are milled in the cylinder jacket to realize the relationship for focusing the front and the rear lens. The cylinder can be rotated about its axis. Both lenses are placed inside the cylinder with allowance for sliding in axial direction and are carried in the cams of the cylinder with mounting brackets. This construction ensures controlled coupling of the lens intervals which correspond to the above equations.
The fabrication effort for a mechanically compensated optical imaging system of this type is considerable. The cams in the cylinder must be milled with a high degree of finishing accuracy. Even close tolerances in the cams lead to deviations from the desired values of the lens positions and, thereby, to deviations from the desired positions of the image plane, thus leading to pronounced deterioration of the image in the fixed receiving plane.